Wheel‑sets bear the full vertical load, lateral impact and braking forces from the carbody. Their assembly quality has a direct bearing on train operation safety. Thermal shrink‑fitting between wheels and axles is prohibited; cold‑press interference fit is adopted uniformly. The complete wheel‑set production workflow consists of manufacturing and inspection of individual wheel and axle components, followed by selective fitting and press‑fitting, and final inspection of the assembled unit to obtain qualified wheel‑set products.
Preparation of Wheel and Axle Components
Wheel Preparation
Roll‑formed steel wheels go through smelting, forging‑rolling forming, local tread quenching heat‑treatment, and CNC machining for finish‑machining of the hub bore, wheel tread and wheel flange. Subsequent non‑destructive testing including ultrasonic testing and magnetic particle inspection is carried out, together with full dimensional verification. Wheels passing all inspections are kept as finished wheel blanks awaiting assembly.
Axle Preparation
Axles are made of alloy forged steel. After forging of steel billets, overall quenching‑and‑tempering heat‑treatment is performed. Lathe turning and grinding produce wheel seats, axle body and axle journals. The wheel seat is the critical mating surface for the wheel hub; its cylindricity and surface roughness are subject to stringent control. Upon completion of machining, the axle is tested by ultrasonic and magnetic particle inspection to eliminate crack defects originating from forging and machining.
Once component inspections are fully passed, wheels and axles are transferred to the wheel‑set assembly station.
Selective Fitting of Components
Selective fitting is conducted prior to press‑fitting. The actual diameters of wheel hub bores and axle‑end wheel seats are precisely measured respectively, and a proper interference value is determined in accordance with technical specifications.
Interference is the core assembly parameter. Excessive interference will induce excessively high assembly stress on the inner hub wall, which may lead to radial cracking during service. Insufficient interference may result in hub loosening and slippage under vehicle vibration and impact. After selective fitting, component serial numbers are recorded to establish original wheel‑set history files. Before assembly, hub bores and wheel‑seat surfaces are cleaned and coated evenly with specified lubricant to prevent scuffing of mating surfaces in the press‑fitting process.
Cold Hydraulic Press‑Fitting of Wheels
Cold‑press assembly is implemented on a microcomputer‑controlled dedicated wheel‑set press; heating wheels for thermal shrink‑fitting is forbidden.
During press‑fitting, coaxial alignment between the press piston and axle centreline must be guaranteed, with a stable press‑in speed. The hydraulic press slowly pushes the wheel home against the locating end face of the wheel seat. Throughout the whole press‑fitting process, the equipment automatically and continuously records the force‑displacement curve.
The press‑fitting curve serves as the core criterion for assembly acceptance. A qualified curve shall be smooth and continuous without abrupt rises, drops or sharp jumps. If abnormal curve behaviour occurs, work shall be stopped immediately. The wheel shall be unloaded and removed to inspect for scuffing on the hub bore. A wheel with a damaged bore must be scrapped directly and shall not be reused after reconditioning. After one‑side wheel press‑fitting is finished, the axle is flipped over to perform press‑fitting for the opposite‑side wheel.
Inspection of Assembled Wheel‑Set
After both wheels are pressed into place, quality inspection is performed for the complete wheel‑set assembly.
First, geometric dimensions including back‑to‑back wheel distance and wheel position offset are measured; all values shall comply with railway technical standards. Next, reassembly non‑destructive re‑inspection is carried out to check for assembly‑induced cracks on wheels and axles. For high‑speed vehicle wheel‑sets, static balance testing is additionally required to eliminate mass eccentricity and avoid excessive in‑service vibration. Tread‑to‑tread resistance testing is also completed. Once all inspections are passed, unique wheel‑set identification marks are stamped.
Subsequent Assembly of Bearings and Brake Discs
After obtaining the basic wheel‑set by wheel press‑fitting, rolling bearings are press‑fitted onto axle journals at both axle ends. For disc‑brake‑equipped vehicles, axle‑mounted brake discs are also press‑fitted. Bearing press‑fitting likewise records force‑displacement curves and implements selective bearing matching. Upon completion of full wheel‑axle assembly, anti‑corrosion protective paint is applied. Finished products are put into storage for bogie final assembly.
Brief Overview of Wheel‑Set Maintenance Process
After wheel‑sets enter revenue service, they are taken down for overhaul when reaching maintenance intervals. Bearings are removed first, then wheels and axles undergo repeated crack detection. Worn or damaged wheel treads are restored to original contour by wheel turning. Wheels with cracks exceeding permissible limits or axles suffering fatigue cracks shall be scrapped. After overhaul, bearings are selectively refitted and press‑mounted. Complete maintenance records and service history are retained for overhauled wheel‑sets as well.
Conclusion
Wheel‑set technology constitutes an integrated process system covering component manufacturing, selective fitting, hydraulic press‑fitting and final‑unit inspection. Metallurgical and machining quality of individual components forms the foundation. Proper interference selection and process monitoring via force‑displacement curves during press‑fitting are the key points of assembly technology. Final‑unit inspection and traceable service records provide the final‑stage quality safeguard. Rigorous implementation of the full set of process specifications ensures long‑term operational reliability of wheel‑sets in service.
